Bistability is a well known phenomenon, and the hysteresis of magnetic materials in order to manufacture bistable memories, has been long since exploited. The phenomenon is characterized by the existence of two values of an output quantity (corresponding to material saturation and relaxation, respectively) for the same value of an input variable, the attainment of either output value depending on the direction in which the input value is made to vary.
More recently the same phenomenon has been observed in optical devices (interferometers) made of materials with nonlinear properties, i.e. materials in which certain intrinsic parameters (such as refractive index and absorption constant) depend on the optical power inside the device. More particularly, in most of the materials of interest for optical communication, nonlinear refractive index can be expressed as the sum of a constant term and of a term depending on the power I of the signal in the device, according to relation EQU n=n0+n2.multidot.I
where n0 is the linear refractive index (which is constant), while n2 is the so-called nonlinear refractive index coefficient.
Present interest in optical communication systems, which allow much higher speeds than electronic systems, has led to proposals to exploit optical bistability in the implementation of digital memories or logic elements and circuits capable of replacing as far as possible the electronic components in such systems. Optical memory devices for use e.g. in optical switching and processing systems have been widely described in the literature, such devices using active or passive Fabry-Perot interferometric cavities.
For correct use of one of these devices, its characterization also from the bistability standpoint will be necessary and more particularly, the nonlinear refractive index coefficient must to be determined. Various methods are known for measuring such a coefficient in the nonlinear material from which the device is made. The simplest method is based on interferometric techniques and is described by D. Milam and M. J. Weber in a paper entitled "Measurement of nonlinear refractive index coefficient using time-resolved interferometry: Applications to optical materials for high-power neodymium lasers", Journal of Applied Physics, Vol. 47, 1976, pages 2497 and ff.
According to this method a sample of the material is introduced into an interferometer branch, a variable-intensity light beam is launched into the sample so as to cause refractive index variation, and the interference fringe shifts due to such index variations are measured to obtain n2. A correct evaluation of the positions of the visibility maxima and minima requires accurate digital processing of the experimental data to eliminate the noise present in the measurement.
However, measuring n2 directly in a device under operating conditions could be more significant, since it is to be presumed that the nonlinear refractive index coefficient of a material, like the linear refractive index, may be modified when incorporating the material into a device. The above method could theroetically be applied also to measure coefficient n2 of a device in addition to that of the material, yet such a measurement would require a more complex experimental system than used in practical applications of the devices (bistable lasers) under test.
A method of characterizing bistable semiconductor lasers has already been suggested by the Applicant (EP-A 0 343 610, published on Nov. 29, 1989), where the nonlinear refractive index coefficient is measured in the device under operating conditions, and the same equipment as used in the practical application of the device (e.g. in signal regeneration and/or amplification systems) is used for the measurement. According to that method, the laser output power is measured as a function of the input power, to determine the hysteresis loop of said the output power; the switching points between the two stable states of the laser are identified by using the power values measured; the output and input power values relevant to such points are memorized; and the value of the nonlinear refractive index coefficient of the nonlinear material which is used for manufacturing the laser is obtained from the output and input power values relevant to at least one of the points.
The method proposed has, however, some disadvantages depending on the necessity of identifying the switching points; actually, the transition regions including such points are defined not only by the optical behavior of the material, but also by the electrical characteristics of the components by which the hysteresis loop is detected; in addition these transitions are rather unstable in time, that is why repeated averages of the values obtained are to be computed. As a consequence switching point positions can be evaluated only within a certain range, and not precisely.